It’s always going to be subjective (bradrh) to some extent, partly because these surfaces are variable. There’s a byway near me that has seen some gravel in the past, and is fine for road bikes in dry weather. Even a summer downpour turns bits of it into a mudbath. It’s currently tagged “tracktype=grade3” though grade4 would be more appropriate in winter. I’ve changed “surface=unpaved” to “dirt”, that being the dominant surface from a user point of view (do we have too many surface types?) and set “smoothness=bad” based on the wiki.

When reviewing the smoothness levels I see a discrepancy from the point of view of someone on 2 wheels - “intermediate” includes some pretty awful asphalt, while good unpaved roads are supposed to be “bad” but can be far smoother than damaged paving

I agree broadly with your take. I might push things a little further, by taking my tourer down stuff better suited to a gravel bike, or even round the local blue MTB trail, but me being odd is an edge case.

I plan using Komoot, with OSM (mainly cycle mode but flipping to standard view) almost always open when routing in unfamiliar areas, along with other sources of info. Komoot certainly makes good use of OSM track quality data, though how it uses each field is a bit opaque. Of particular interest to me, “touring” mode routes down gravel far more willingly than “road cycling” mode, as well as towpaths etc as you’d expect. It can be rather optimistic where a RoW runs through a farm, so I’ve ended up on slippery lumpy grass on slick tyres more than once Often there’s a lack of data for it to go on, something we can help with.

My MTB (entry-level hardtail) planning tends to be less automated, and some hike-a-bike is to be expected, thus I’m less aware of what Komoot does in that mode.

One stretch I did recently was tracktype=grade2, surface=gravel (NCN 78/Great Glen Way through Clunes Forest) with no “smoothness”. Where they’d been logging “smoothness=very bad” would have been appropriate (just about got through on the tourer without walking but standing on the pedals); adjacent sections just “bad”. Other nearby gravel sections of NCN78 would have to be “intermediate” smoothness per the wiki though honestly you could go flat out on 23mm tyres if not for walkers and dogs, so “good” wouldn’t be wrong.

I’ve updated some sections (and may be able to do a few more before I forget my recent trip) but am wary of making a judgement on mtb:scale so tend to concentrate on surface material, tracktype and smoothness. Plus on long rides there’s a limit to how much surveying I can do - I often rely on remembering numbers from my tripmeter with a single keyword, until I stop anyway to write things down.

Regarding max gradient on mtb:scale: When I have used mtb:scale, I’ve tagged for the most severe features encountered - so plenty of mtb:scale=2 despite not going over 40% steepness. The steepness numbers do seem too high and inconsistent with mtb:scale:uphill; perhaps the description and examples came from an experienced downhiller.

One thing that would be nice to see is an (up-to-date) article on all the various surface type/quality fields for cyclable ways.